For years, Alan Lloyd has regarded diesel as a dirty word, synonymous with brown haze and cancer-causing black soot. It's a view he has shared with environmental activists across the U.S.

But in a striking change of heart that could alter the kinds of cars and trucks Americans drive, the chairman of the powerful California Air Resources Board is taking a new look at diesel vehicles. He thinks they're poised to emerge as part of the solution to a different environmental problem that's gaining more attention in the U.S.: global warming.

Coming from the head of California's famously pugnacious clean-air agency, that amounts to environmental apostasy. In the decades following World War II, California was a main instigator of the world's fight against smog, and it has waged that battle aggressively ever since. CARB's mandates for pollution cuts in everything from gas cans to lawnmowers to 18-wheelers have been celebrated by environmentalists, criticized by industry and mimicked by national governments from Washington to Europe.

Nowhere has CARB been more aggressive than in its campaign to clean up automobiles -- a priority that reflects California's position as the nation's biggest single auto market, accounting for 12 percent of U.S. sales. Over the years, CARB's edicts have often shaped Environmental Protection Agency policy and thus the way Detroit designs cars.

But now, Dr. Lloyd is being forced to address the issue of global warming, and here, diesel engines are the greener option because they don't pump out as much so-called greenhouse gas as gasoline engines do. Diesels still aren't as clean as their gasoline-powered cousins in terms of smog pollutants. But Dr. Lloyd says he has concluded that a new generation of high-tech diesels developed for Europe bears little resemblance to the smoke-spewers that Americans remember from the 1970s and 1980s. He says he thinks it's possible that within five years -- tomorrow in the world of cars and trucks -- the auto industry will have bridged that gap.

"Ten years ago, I wouldn't have believed what I'm telling you now," says Dr. Lloyd, who in the past several weeks has begun a series of closed-door meetings with auto-industry officials to discuss several clean-car technologies. "However, we have confidence that, given past history, the auto industry will rise to the challenge, and we will have light-duty diesel in the U.S. and California."

Dr. Lloyd isn't the only environmental official reassessing diesel. Earlier this year, the EPA tested a new version of a diesel car from Toyota Motor Corp. that's under development for future sale in Europe. The agency concluded that the car already meets a round of tough new anti-smog standards that are set to phase in between 2004 and 2007 in the U.S. EPA officials are scheduled to explain those test results today at an auto-industry conference in San Diego. And they expect to test more diesel cars, as well as sport-utility vehicles, from other manufacturers before the end of the year.

"Clean diesel sounds like an oxymoron," says Margo Oge, director of the EPA's office of transportation and air quality. "It's not."

Not everyone is so optimistic that the technology to make diesel engines as clean as gasoline engines will fall into place. No one knows, for instance, whether the Toyota tested by the EPA will stay clean enough as it ages to comply with the new antismog rules. But the progress in diesel engines is setting the stage for yet another fight in California between green activists and auto makers. And this time, caught in a shift in the environmental movement's priorities, CARB finds itself in the uncomfortable spot of having to negotiate with the auto industry it has long ordered around.

That's because a new California law requires that the agency now address automobiles' effects on global warming, not just on smog. The means of fighting these two very different environmental enemies aren't always compatible.

Smog is the foe CARB was founded to fight, and diesel engines produce more smog-causing pollutants than gasoline engines do, which is why diesel has been anathema to U.S. environmentalists.

But there's another environmental concern on the horizon. Carbon dioxide, called a greenhouse gas because of mounting evidence that it contributes to increases in the temperature of the earth's atmosphere, is produced when any fossil fuel burns. Limiting carbon-dioxide emissions requires burning less fuel. And diesel engines require less fuel to produce a given amount of energy than gasoline engines do.

Detroit's Big Three and their European and Japanese rivals face growing pressure to make their vehicles more fuel-efficient to reduce dependence on Middle East oil and help slow global warming. Though the U.S. has said it won't ratify the Kyoto treaty to curb global warming, the specter of the California automotive greenhouse-gas law -- the first in the nation -- and the likelihood of tougher federal fuel-economy standards have the auto industry scrambling to make its vehicles more efficient. As "light trucks," a category that includes SUVs, pickup trucks and minivans, have soared in popularity in the U.S., they've dragged down the average fuel economy of the fleet to the lowest level in two decades.

The industry argues that esoteric technologies such as battery-powered vehicles are impractical and won't sell. With increasing frustration and urgency, auto makers are making the pitch to American regulators that a smarter response to the country's fuel-consumption problem can be found in the success of diesels across the Atlantic.

In Europe, tax policies have favored diesels for decades as part of a broader push for energy efficiency. By the time Europe began regulating smog-causing auto emissions about 30 years ago, diesel was entrenched, so the Continent wrote its rules to prod the industry to clean up the technology -- not outlaw it. Today's diesel vehicles in Europe are cleaner and quieter than their predecessors, though still not as clean and quiet as gasoline models. They're also often more fun to drive. Diesel power now commands about one-third of Western Europe's new passenger-car market, and most industry analysts expect that share to grow.

In the U.S., where gasoline is relatively cheap, diesels never caught on, except in commercial vehicles. Today diesel accounts for less than 1 percent of U.S. car sales.

But that could change as pressure mounts on Dr. Lloyd and other environmental regulators to go after greenhouse gases. In 1998, the year before Dr. Lloyd joined CARB's board, the agency approved the new anti-smog regulations that are set to take full effect by 2007, tailoring those rules to try to keep out dirty diesels. A year later, in 1999, the EPA followed suit with similar regulations for the rest of the country.

Now CARB is on the defensive. The most famous part of its antismog effort, its "zero emission vehicle" mandate, written in 1990, is floundering. Battery-powered cars haven't electrified consumers, and fuel-cell-powered vehicles in significant numbers remain at least a couple of decades away. Auto makers already have sued to block the zero-emission-vehicle mandate, and they're threatening a similar suit against the greenhouse-gas law.

In this changed climate, Dr. Lloyd is signaling support for the auto industry's diesel push. He vows he won't relax the impending anti-smog rules to let in diesel engines that aren't as clean as gasoline versions, which some auto makers have advocated. What he will do, he says, is publicly resist calls by some environmentalists to make those rules more severe than he believes public health demands.

"I hear from some of my colleagues in the environmental community that if it's diesel, by definition it's dirty," says Dr. Lloyd. He says he already has received emails from activists and regulators criticizing his new pro-diesel tone. "I'm demonstrating that I'm willing to look at the facts and be convinced of those facts." He adds: "It's a tough sell."

The CARB chief says that if auto makers can produce diesels that meet the latest anti-smog standards, and if further study confirms that those standards are adequate to protect public health, it would be unfair to toughen the rules yet again just to try to keep diesel technology out. That sets the stage for a fight between him and some environmentalists who say even the upcoming standards don't adequately target particularly tiny diesel-soot particles that some new studies indicate pose a particular health risk. Diesel particulates are believed to cause cancer, and they can aggravate respiratory problems.

"I think there are legitimate public-health questions around diesels whether or not they meet the standards," says Jason Mark, who works in Berkeley, Calif., as director of the clean-vehicles program at the Union of Concerned Scientists, an environmental group based in Cambridge, Mass.